In the fight against stock pests such as insects and rodents, there are used today, on a very considerable scale, agents which on exposure to moisture from the atmosphere or from grain, evolve highly active phosphin. Agents containing alkaline earth or earth metal phosphides have been recognized as being particularly suitable; phosphides of calcium, magnesium and aluminum have been found to be particularly advantageous.
Phosphin, however, tends to self-ignite, and this characteristic requires special precautions for ensuring substantially complete safety in the use of these agents. Up to now attempts were made to meet this requirement by control of the velocity of formation of the phosphin and/or suppression of its tendency to self-ignite.
These attempts pursued a variety of approaches. Thus it is known to use phosphide containing pesticides in special water-tight packages which, however, are permeable to gas and moisture. In practice, however, this proposal has proven effective only in certain special fields of application.
Other methods which have been suggested, operate by rendering phosphide particles, or groups of such particles, hydrophobic by means of water-repellent substances. Among the materials contemplated for this purpose are paraffins, waxes, stearates, silicones and artificial resins. Another known method involves the embedding of the phosphides in plastics.
The afore-mentioned procedures are designed to impede, by control of the velocity of formation of phosphin, the generation of phosphin-air mixtures with their tendency to self-ignite. If in accordance with these known methods, the phosphide is present in hydrophobic form, there is indeed an influence on hydrolysis, but an adequate protection against self-ignition of the phosphin generated is not ensured in this manner.
Also known is a proposal to suppress the tendency of phosphin to self-ignite, by the addition of thermally readily decomposable substances. Preferred among the substances employed for this purpose, are those which split off CO.sub.2, and partly NH.sub.3, such as sodium carbonate, sodium bicarbonate, ammonium carbonate, ammonium bicarbonate, and in particular ammonium carbaminate. It is also known to admix to the phosphides, certain readily volatile organic substances the latent heat of which surrounds the phosphide particles with a cold zone designed to prevent that heat accumulation apt to trigger the auto-ignition of the phosphin.
A reliable protection from self-ignition of the phosphin, and the assurance of safety in the handling of the phosphide, however, is not accomplished in this manner. It is in the nature of a thermally readily decomposable or readily volatile substance that its effective life is limited; this is particularly true if the phosphide to be protected in this manner, is to be used at relatively elevated temperatures. Inasmuch as the formation of hydrogen phosphide, or phosphin, is an exothermic process, such a rise in temperature may occur even in areas having a moderate climate.
A relatively recent publication recommends the addition to the phosphides, of oxygen or nitrogen containing organic substances which are supposed to inhibit the auto-ignition of the phosphin. Tests have demonstrated, however, that the inhibitors recommended fail to be effective to an acceptable degree. Moreover, not a small part of the substances recommended are quite unsuitable for toxicological reasons, while others are far from being readily available.